Nine people were killed and over 100 wounded on Saturday when a bomb ripped through a mosque in Iran's southern city of Shiraz, in an unprecedented strike on a non-frontier Iranian urban centre, officials said.
The massive bomb explosion in the men's section of the mosque took place at around 9:00 pm (1630 GMT) during an evening prayer sermon by a prominent local cleric, the Fars news agency quoted local officials as saying.
"One hundred and five people were wounded and nine were killed in the bomb attack in Shiraz," the agency quoted a hospital source as saying, adding all the city's medical facilities had been mobilised to deal with the casualties.
Fars said that the death toll was set to rise as many of the victims were in a critical condition. There have been deadly attacks in Iran's border cities with ethnic minority populations in recent years but such a strike in a city such as Shiraz is unprecedented in recent decades.
The normally placid city is not in a border zone, nor is it home to any significant population of ethnic or religious minorities.
"Around 9:15 pm, after the sermon, the sound of an explosion resounded in the section reserved for men and a cloud of dust billowed up to the sky," witness Saideh Ghorbani, 20, told the agency.
Iranian state television said the explosion caused "several dead and wounded", without specifying the number of casualties. English language state channel Press-TV said that the bomb may have been placed in a handbag.
"There was a huge blast and the whole place lit up. Everyone started shouting and screaming and tried to help each other," another witness, Marzian Mohammadnejad, told Press-TV.
There were no reports of any group claiming the blast and no further details were given on how the explosion could have been set off.
"A report has been opened into the causes of the blast," said Mohammad Reza Hadaegh, the deputy governor of Fars province in charge of security issues, according to the official IRNA news agency.
Deadly attacks in Iran have become extremely rare events in the past two decades, although the first years after the 1979 Islamic revolution were marked by a succession of bomb blasts in Tehran by outlawed opposition groups.
The last major attack in Iran was a February 2007 strike by suspected Sunni rebels in the city of Zahedan in the southeastern Sistan-Baluchestan border province that killed 13 members of the elite Revolutionary Guards. That attack was the deadliest such strike to have hit the Islamic republic in years.
There were also deadly attacks in 2005 and 2006 in the southwestern city of Ahvaz which has an Arab population and Iranian officials blamed them on Britain and Arab separatists. There has also been unrest in northwestern Kurdish-populated provinces.
But despite sharing borders with conflict-torn Iraq and Afghanistan, security in Iran is normally considered to be among the most stable in the Middle East.
The local cleric, named only as Hojatoleslam Anjavinejad, had been preaching against Wahhabism -- the ultra-conservative form of Sunni Islam practiced in Saudi Arabia.
In his sermon, he had also attacked Bahais, a group who were once by far Iran's biggest non-Muslim minority and believe in the unity of all religions, Fars reported.
However, they are deemed as apostate by the Islamic republic and their beliefs are not recognised by the constitution. The target of the attack appeared to be the Rahpouyan cultural centre that is part of the mosque, located in a residential quarter in the centre of the city, reports said.
Shiraz is one of Iran's most famous cities and a popular destination for foreign tourists due to its proximity to important ancient sites from the Achaemenian Empire that ruled much of Asia from 550-331 BC.
Iran has repeatedly accused its Western enemies of seeking to stir trouble in the country at a time of mounting tension over its contested nuclear programme.
Officials have frequently spoken of arrests and the breaking up of cells who were planning to stage attacks in the country.